A number of methods of encoding and decoding audio signals have been developed up to now. Particularly, in these days, IS13818-7, which is internationally standardized in ISO/IEC, is publicly known and highly appreciated as an encoding method for reproducing high quality sound with high efficiency. This encoding method is called Advanced Audio Coding (AAC). In recent years, the AAC is adopted to the standardization called MPEG 4, and a system called MPEG-4 AAC that has some extended functions added to the IS13818-7 has been developed. An example of the encoding procedure is described in the informative part of the MPEG-4 AAC.
The following is an explanation for an audio encoding device using the conventional encoding method referring to FIG. 1. FIG. 1 is a block diagram that shows the structure of a conventional encoding device 100. The encoding device 100 includes a time-frequency transforming unit 101, a spectrum amplifying unit 102, a spectrum quantizing unit 103, a Huffman coding unit 104 and an encoded data stream transfer unit 105. A digital audio signal on the time axis obtained by sampling an analog audio signal at a predetermined frequency is divided into every predetermined number of samples at a predetermined time interval, transformed into data on the frequency axis through the time-frequency transforming unit 101, and then given to the spectrum amplifying unit 102 as an input signal into the encoding device 100. The spectrum amplifying unit 102 amplifies a spectrum included in every predetermined band with one certain gain. The spectrum quantizing unit 103 quantizes the amplified spectrum with a predetermined transform expression. In the case of AAC method, the quantization is conducted by rounding off frequency spectral data, which is expressed in floating points into an integer value. The Huffman coding unit 104 encodes the quantized spectral data in a set of certain pieces thereof according to Huffman coding, and encodes the gain in every predetermined band in the spectrum amplifying unit 102 and the data that specifies the transform expression for the quantization according to Huffman coding, and then transmits the codes of them to the encoded data stream transfer unit 105. The Huffman-coded data stream is transferred from the encoded data stream transfer unit 105 to a decoding device via a transmission channel or a recording medium, and reconstructed as an audio signal on the time axis by the decoding device. The conventional encoding device operates as described above.
However, in the conventional encoding device 100, a capability for compressing data amount depends on the performance of the Huffman coding unit 104 or the like, so when the encoding is conducted at a high compression rate, that is, with a small amount of data, it is necessary to increase the gain sufficiently in the spectrum amplifying unit 102 and encode the quantized spectrum stream obtained by the spectrum quantizing unit 103 so as to make it a smaller amount of data in the Huffman coding unit 104. According to this method, if the encoding is carried out for making an amount of data smaller, the frequency bandwidth for reproduced sound and music practically becomes narrow. Therefore, it cannot be denied that the sound and music would be furry for human hearing. As a result, it is impossible to maintain the sound quality. That is a problem.
Also, within the conventional encoding device 100, the input signal expressed on the time axis is transformed into the frequency spectrum expressed on the frequency axis by each predetermined interval (the number of samples) in the time-frequency transforming unit 101. Therefore, the signal quantized for the encoding in this latter stage is the spectrum on the frequency axis. It is inevitable for a quantizing process to have some quantization errors through processing such as rounding off a decimal value in the frequency spectral data into an integer value. On contrary to a fact that assessment of the quantization error generated in the signal is easy on the frequency axis, it is difficult on the time axis. Because of this, it is not easy to improve time resolution ability of the encoding device through the assessment of the quantization error reflected on the time axis. Also, if the amount of data available to allocate to the encoding is sufficient, it is possible to improve both frequency resolution ability and time resolution ability. But if the amount of data allocated for the encoding is small, it is extremely difficult to improve both.
In view of the above-mentioned problem, the present invention aims at providing an encoding device, capable of encoding an audio signal at a high compression rate with an advanced level of the time resolution ability, and a decoding device capable of decoding frequency spectral data in a wide band.